Ascomata: first immersed in the host thallus, then bursting through the cortex to become somewhat elevated, slightly convex, rounded, black but partly covered by a rusty brown pruina, developing irregular cracks and fissures in the layer above the hymenium which later breaks into separate parts; disc finally widely exposed, irregular, umbilicate or roundish, entirely covered by a rusty brown pruina, at the edges often with remnants of the layer formerly obscuring the disc; up to 2.5 mm diam.; not visibly damaging the host thallus, not inducing galls stroma: ± multilocular; sterile stromatic tissue dark brown, K+ dark olivaceous, N+ reddish brown; pigment granulose (Atrabrown) hymenium: brownish, I+ red, c. 100 µm high; crystals overlaying the hymenium (forming the pruina) orange, K+ purplish red, dissolving (crystals absent in pol. light), N+ bright yellow, crystals not dissolving paraphyses: richly branched and anastomosing, 2.5-3 µm thick, apically not distinctly enlarged asci: broadly cylindrical, 6- or 8-spored, 80-90 x 20-25 µm ascospores: hyaline, 3-septate, 19-25 x 6-8 µm; perispore distinct, hyaline, c. 1 µm thick, becoming brownish and slightly granulose when overmature Pycnidia: immersed in ascoma conidia: dimorphic in the same conidial cavity: microconidia bacilliform, attenuated at lower end, aseptate, 5-6 x 1-1.5 µm; macroconidia bacilliform, more rarely slightly clavate, 1-septate, (12-)15-17 x 4-5 µm. Host: on the thallus of Niebla robusta World and Sonoran distribution: known only from the type specimen in California (West Anacapa Island). Notes: Plectocarpon leuckertii and P. nashii are the only species of the genus to have ascomata covered by an orange to reddish pruina. Plectocarpon leuckertii differs in having narrower ascospores (4.5-5.5(-6.5) µm), shorter asci (65-80 µm), frequently lirellate ascomata and different hosts (Pseudocyphellaria intricata and Ps. norvegica).